参数资料
型号: TC1305
厂商: Microchip Technology Inc.
英文描述: Dual 150mA CMOS LDO With Select Mode⑩ Operation, Shutdown and Independent RESET Output
中文描述: 双150mA的CMOS低压与选择模式⑩行动,关闭和独立RESET输出
文件页数: 7/18页
文件大小: 530K
代理商: TC1305
2002 Microchip Technology Inc.
DS21526A-page 7
TC1305
5.0
THERMAL CONSIDERATIONS
5.1
Thermal Shutdown
Integrated
regulator off when die exceeds approximately 160°C.
The regulator remains off until the die temperature
drops to approximately 145°C.
thermal
protection
circuitry
shuts
the
Thermal shutdown is intended to protect the device
under transient accidental (fault) overload conditions.
Thermal Shutdown may not protect the LDO while
operating above junction temperatures of 125°C
continuously. Sufficient thermal evaluation of the
design needs to be conducted to ensure that the
junction temperature does not exceed 125°C.
5.2
Power Dissipation
The amount of power the regulator dissipates is
primarily a function of input and output voltage, and
output current. The following equation is used to
calculate worst case
actual
power dissipation.
EQUATION 5-1:
The
(Equation 5-2) is a function of the maximum ambient
temperature (T
A
MAX
), the maximum allowable die
temperature (125°C), and the thermal resistance from
junction-to-air (
θ
JA
). The MSOP-10 package has a
θ
JA
of approximately 113°C/W when mounted on a four
layer FR4 dielectric copper clad PC board.
maximum
allowable
power
dissipation
EQUATION 5-2:
Equation 5-1
Equation 5-2 to ensure regulator thermal operation is
within limits. For example:
can
be
used
in
conjunction
with
Given:
V
IN
MAX
V
OUT1
MIN
= 3.0V ± 2.5%
V
OUT2
MIN
= 3.0V ± 2.5%
I
LOAD1
MAX
= 120mA
I
LOAD2
MAX
= 120mA
T
J
MAX
= 125°C
T
A
MAX
= 55°C
θ
JA
= 113°C/W
Find: 1. Actual power dissipation
2. Maximum allowable dissipation
= 3.8V ± 5%
Actual power dissipation:
P
D
[(V
IN
MAX
– V
OUT1
MIN
)] x I
LOAD1
MAX
+ [(V
IN
MAX
– V
OUT2
MIN
)] x I
LOAD2
MAX
[(3.8 x 1.05) – (3.0 x .975)] x 120 x 10
-3
+ [(3.8 x 1.05) – (3.0 x .975)] x 120 x 10
-3
= 256mW
Maximum allowable power dissipation:
In this example, the TC1305 dissipates a maximum of
256mW; below the allowable limit of 620mW. In a
similar manner, Equation 5-1 and Equation 5-2 can be
used to calculate maximum current and/or input
voltage limits. For example, the maximum allowable
V
IN
is found by substituting the maximum allowable
power dissipation of 620mW into Equation 5-1, from
which V
IN
MAX
= 5.6V.
5.3
Layout Considerations
The primary path of heat conduction out of the package
is via the package leads. Therefore, layouts having a
ground plane, wide traces at the pads, and wide power
supply bus lines combine to lower
θ
JA
and therefore
increase the maximum allowable power dissipation
limit.
Where:
P
D
(V
IN
MAX
– V
OUT1
MIN
)I
LOAD1
MAX
+
(V
IN
MAX
– V
OUT2
MIN
)I
LOAD2
MAX
P
D
V
IN
MAX
V
OUT1
MIN
I
LOAD1
MAX
V
OUT2
MIN
I
LOAD2
MAX
= Worst case actual power dissipation
= Maximum voltage on V
IN
= Minimum regulator output voltage1
= Maximum output (load) current1
= Minimum regulator output voltage2
= Maximum output (load) current2
P
D
MAX
= (T
J
MAX
– T
A
MAX
)
θ
JA
Where all terms are previously defined.
P
D
= (T
J
MAX
– T
A
MAX
)
θ
JA
= (125 – 55)
113
= 620mW
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